The invention relates generally to engines, and more particularly, to a system and method for controlling a substitution rate in an engine, for example, a dual fuel engine.
In a compression-ignition engine, such as a diesel engine, a fuel injection system injects fuel (e.g. diesel fuel) into compressed air within each of the engine cylinders to create an air-fuel mixture that ignites due to the heat and pressure of compression. Unfortunately, engine efficiency, power output, fuel consumption, exhaust emissions, and other operational characteristics may be less than ideal. In addition, conventional techniques to improve one operational characteristic often worsens one or more other operational characteristic. For example, attempts to decrease specific fuel consumption often cause increase in various exhaust emissions. Engine exhaust emissions including pollutants such as carbon monoxide, nitrogen oxides (NOx), particulate matter (PM), and unburned hydrocarbons (UHC) are generated due to imperfect combustion of fuel within the combustion chamber. The amount of these pollutants varies depending on the fuel-air mixture, compression ratio, injection timing, ambient conditions, and so forth.
In the oil and gas market and transportation sector, for example, expenditure on fuel is one of the main contributors to the total cost of operation. The rapid expansion and abundance of natural gas in some areas of the world is driving a dramatic cost advantage of natural gas over diesel fuel, making natural gas a very economical fuel source. A dual fuel engine is based on a traditional diesel engine, with the addition of dual fuel specific hardware. When the engine is operating in dual fuel mode, natural gas is introduced into its intake system. Near the end of the compression stroke, diesel fuel is then injected. The diesel fuel ignites and the diesel combustion causes the natural gas to burn. In one example, around 40 percent diesel is injected and sixty percent natural gas is injected.
The substitution rate is defined as the fraction of the total fuel energy that is provided by the gaseous fuel, for example, natural gas. The maximum substitution rate for a dual fuel engine is generally limited by knock and fast combustion which are each driven by in-cylinder temperatures at the start of combustion. Specifically, the substitution rate is often limited by the rate of combustion for a given operating condition. In other words, for many cases, the rate of combustion can be rapid if the substitution rate is substantially increased.
There is a need for an enhanced system and method for controlling substitution rate for an engine in order to control the rate of combustion.